1. Field of the Invention
The invention relates to a signal processing apparatus and method for processing an image signal of an image pickup apparatus such as a digital camera or the like.
2. Related Background Art
FIG. 2 shows a construction of a conventional signal processing apparatus using image pickup devices of a complementary color system. Color filters of four colors of Mg (magenta), G (green), Cy (cyan), and Ye (yellow), as shown in FIG. 3, are adhered to the image pickup devices for each device, in order. An output signal from the image pickup device is transmitted through an OB (Optical Black) circuit 201, an individual difference variation correction circuit (pixel gain circuit) 202, a WB (White Balance) circuit 203, and an offset circuit 204 and separately supplied to a luminance signal formation processing system and a chroma signal formation processing system.
In the luminance signal formation processing system, a gain level difference which is caused by the color filters is removed by a low pass filter (notch circuit) 215 and a clamping process is performed to a luminance signal by a Y clamp circuit 216. Subsequently, an edge emphasis is performed by an APC (aperture) circuit 217. A level correction by color difference signals is performed by a Y compensation circuit (Ycomp circuit, luminance signal compensation circuit) 218. Thus, the luminance signal becomes a Yh luminance signal of 8 bits by a Y gamma (Y-Gamma, luminance signal gamma correction) circuit 219 having conversion characteristics as shown by a curve 502 in FIG. 5.
In the chroma signal formation processing system, ti the signal is interpolated by a color interpolation circuit 205 as if there were values in all pixels of four colors. A conversion of (the complementary colors →pure colors→color difference) is performed by a color matrix circuit (color conversion circuit) 206. A subtle correction of a chroma signal is performed by a linear clip matrix circuit 207. Subsequently, a gain of chroma signal in a saturation luminance area is suppressed by a C-SUP (chroma suppression) circuit 206. A band of the chrome signal is limited by a low pass filter 209. After that, a saturation is adjusted by a chroma gain circuit 210. Thereafter, the chroma signal is again converted into RGB signals by a matrix circuit 211 by a low band Yl signal and the color difference signals.
The low band ROB signals become RGB (red, green, blue) signals of 8 bits by a C gamma (chroma signal gamma correction) circuit 213 having the conversion characteristics as shown by the curve 502 in FIG. 5. Subsequently, the RGB signals are converted into color difference signals Cr and Cb by a color conversion circuit 214.
An image signal is formed by the Yh signal from the luminance signal formation processing system and the Cr and Cb signals from the chroma signal formation processing system.
However, when the luminance signal and the chroma signal are gamma converted into signals having the same number of bits as in the above conventional technique, the following problems occur.
Generally, each of RGB of an output range of a monitor, as an output apparatus, consists of 8 bits, and there are gamma characteristics as shown by a curve 401 in FIG. 4. An output of the camera is a linear signal in which, for example, each of RGB consists of 11 bits. It is, therefore, necessary to non-linearly compress (gamma conversion) the camera output to a signal having the number of bits of the output apparatus, in accordance with the characteristics of the monitor.
Therefore, although linear conversion of a luminance signal is preferable, so as to prevent deterioration of the hue, when linear conversion is performed, an image becomes dark due to influence by the gamma characteristics, of the monitor. To accurately reconstruct the gradations of the luminance, therefore, a reverse gamma conversion of the monitor characteristics as shown by a curve 402 in FIG. 4 (a curve 504 in FIG. 5), is performed. Although there is a slight deviation of the hue, the accurate gradation reconstruction of the luminance is held. If there is such a gamma curve, however, the image becomes an image which lacks contrast in a middle luminance area of a main object.
Hitherto, therefore, a gamma curve which can enhance a contrast of a draw area (middle luminance area) of the main object, as shown by the curve 502 in FIG. 5, is used or a plurality of gamma curves according to the application are selectively used in accordance with a mode. In the case where data of a curve, as shown by the curve 502, is used in order to improve the contrast of the main object, it is necessary to further reduce the gradations in the low luminance area or high luminance area by an increased amount of the contrast in the middle luminance area. In this case, as shown in FIG. 5, particularly in a high luminance and high saturation area of the chroma signal, an output signal difference between R and G in an output of the 8-bit C gamma circuit 213 shown in FIG. 5(a) is smaller than an output signal difference between R and G of the 8-bit C gamma circuit at the timing before the gradations of the middle luminance area in FIG. 5(c) are increased, the saturation is deteriorated, and the output signal difference between B and G is not largely changed, so that the hue remarkably deviates. There are, consequently, drawbacks that occur, such as deterioration in color reconstruction of the high luminance area of the image after gamma conversion, likelihood the image will be white skipped, and discoloration.
An example of the conversion of the chroma signal in the case of a gamma system when the conventional 8-bit C gamma curve 502 is used will be discussed hereinbelow.
<1 >8-Bit C-Gamma
    (a) Before gamma (11-bit RGB)            R=1400        G=1100        B=400            (b) After gamma (8-bit RGB)            R=236        G=228        B=140        Yh=0.3R+0.59G+0.11B=221            (c) RGB-YCrCb conversion            Yl=0.3R+0.59G+0.11B=221        Cr=R−Y=15        Cb=B−Y=−81            (d) MIX with Yh signal on the output apparatus side
Yh = 220R = Cr + Yh = 235B = Cb + Yh = 139G = (Yh − 0.3R − 0.11B)/0.59 = 212
A ratio of the level differences among the signals of RGB remarkably differs as compared with that of the signals before the gamma conversion (refer to FIG. 5(a)). Thus, the image becomes a picture whose hue changes and which is white skipped.